In UV Editor, is there a way to set rotation of selected mesh to 0 (reset/align to view)
You can use UV Squares addon for that.


Enable it in User Preferences by Ctrl+Alt+U > Addons > UV Squares.
If you don't have the addon, save this to your blender-folder/scripts/addons and refresh the list.
Then you can find it in the Image/UV Editor in the left panel T > Misc tab.
Just select the rotated cube unwrap and press To Grid by Shape:




Note: This works only if your UVs are quads.